(1) Field of the Invention
The invention relates to the general field of packaging, more particularly to the packaging of crystal oscillator chips.
(2) Description of the Prior Art
Crystal oscillators derive their function from the piezoelectric effect. Most commonly they are cut from a single crystal of quartz and deform slightly in the presence of an electric field. As illustrated in FIGS. 1a and 1b, quartz crystal 1 is sandwich-coated between two metal electrodes 2 and 3 (normally gold) bonded to a lead frame which is part of a metal can package. The electrodes are used to induce an oscillating electric field in the crystal.
The oscillating electric field produces mechanical strain in the form of a standing wave whose direction depends on the lie of the crystal lattice relative to the surface. In an AT cut quartz wafer, shear oscillations predominate and the mechanical displacements are parallel to the wafer surface. Thus, if the applied electric frequency is equal or close to the natural (mechanical) vibrational frequency of the oscillating wafer (or chip), resonance will occur. An electric oscillator circuit can be designed to lock onto the resonant frequency, making for a highly stable oscillator.
The natural vibrational frequency of a crystal chip depends on a number of factors including thickness, density, and shear modulus, as well as the mass of material that is present on either or both surfaces. The last of these factors is the easiest to change since the mass of the electrodes 2 and 4 can be modified through etching or through additional material deposition. In the production of standard crystals for the communications industry, chips are generally fine tuned by electroplating additional amounts of metal onto the electrodes. The same principle may be used to measure the adsorption of small amounts of material onto a surface. Such a device is known as a Quartz Crystal Microbalance (QCM) and has a mass sensitivity of about 1 kHz per microgram with a theoretical limit as low as 10.sup.-6 micrograms.
Returning to FIG. 1a, a typical package of the current art for a QCM is seen. Quartz chip 1 is held between two metal lead frames 15 each of which makes contact to one of the electrodes 2 or 3 through conductive paste 9. The frames are connected to rods 5 that pass through insulated base 6, becoming pins 7 which are suitable for inserting into an appropriate electrical outlet. Protective metal cap 8 may optionally be placed over the device. This form of mounting is chosen with a view to allowing the crystal maximum freedom of movement in the transverse (shear) mode.
The general principles of the QCM, along with several examples of its application, are discussed in an article by M. R. Deakin and D. A. Buttry "Electrochemical applications of the Quartz Crystal Microbalance" in Analytical Chemistry vol. 61 1989 pp. 1147-1154. The use of an enzyme coating on the surface of a QCM as a specific detector for formaldehyde is discussed in an article by G. G. Guilbault "Determination of Formaldehyde with an enzyme-coated piezoelectric crystal detector" in Analytical Chemistry vol. 55 1983 pp. 1682-1684.